Optical fibre access networks provide communication facilities to subscribers utilizing an optical network of fibres extending from the subscribers premises to a central office of a network provider or operator. Typically a passive optical network (PON) having a point-to-multi-point architecture is used as this is considered the most cost effective solution.
The passive optical network comprises an optical line terminal (OLT) at the central office which is coupled to the point-to-multi-point optical network of fibres. This fibre network uses a number of power splitters to distribute signals via optical fibre to each subscriber, such that an optical signal from the OLT will propagate to optical network terminations (ONT) for each subscriber. Different wavelengths are used for the uplink and downlink optical signals between the passive optical networking OLT and ONT's. Such wavelengths are selected to be outside of the wavelength ranges used in other optical transmission techniques.
For example, typically 1310 nm is used for upstream transmission (from the OLT's to the ONT) and 1490 nm for downstream transmission (from the ONT to the OLT's). To allow compatibility between equipment from different vendors/between different networks, it is important that optical networks conform to standards. Various standards have been agreed for different protocols for operating these PON, including GPON (Gigabit passive optical network—ITU-T G.984) and EPON (Ethernet passive optical network—IEEE 802.3ah). Within GPON, optical signals can be transmitted from the OLT to ONT's at up to 2.5 Gb/s, without requiring optical amplification.
It is widely expected that the next generation of fibre access networks will rely on wavelength divisional multiplexing (WDM) techniques. Various standards have been agreed for WDM systems, including ITU-T G.694.1 & G.694.2 which specify the grids of wavelengths allowed for use in two different WDM systems.
However, it may be some time before WDM optical components become cost competitive. As such, conventional PONs are likely to continue to be installed in the near future. Various arrangements have been proposed for integrating WDM based fibre access networks into existing conventional PON in order to allow for upgrading the capacity of such PON, when cost-effective WDM optical components become available.